A new variant form of phenylketonuria (PKU) was found. This disease is due to a deficiency of the pterin cofactor. An in vivo assay for phenylalanine hydroxylase activity performed on the patient with this form of PKU showed that he has only about 5% of the normal activity. BIBLIOGRAPHIC REFERENCES: Bartholome, K., Byrd, D.J., Kaufman, S. and Milstien, S.: Atypical phenylketonuria with normal phenylalnine hydroxylase and dihydropteridine reductase activity in vitro. Pediatrics 59: 757-760, 1977. Kaufman, S. and Milstien, S.: Phenylketonuria and its variants. Annals of Clinical and Laboratory Science 7: 178-185, 1977.